1: Audio-Technica ATH-M40fs. I've had these for years and done a couple of mods, and they're still my all-around favorite for sound. Very balanced sound. Closed, decent isolation, excellent comfort. Good for every genre of music. Not the strongest sound stage, but an overall miracle for the price ($50 from Amazon). The construction of these is unbelievable and they're a modder's dream. Easy to take apart, every part made for field service, terminal blocks instead of soldering. I still can't believe that someone decided to mass produce something like this, but they're my favorite to experiment with as it's easy to undo changes.
2: Beyerdynamic DT990 Pro 250. Outstanding comfort, wide sound stage, crisp highs. A good bit more expensive than the stuff that I usually pick up out of curiosity, and certainly very nice. Very light, requires a bit of power to drive properly, but lovely when done right. Like #1, I love these for all types of music. Leaks a lot of sound in and out, so a quiet room is required. The coolness and comfort of these much more strongly lends itself to long listening sessions. Sound and comfort would probably put it #1, but the isolation and versatility of the AT keep it ahead for the time being. If this is what ~$150 buys, I'd love to experience $1,000 cans.
3: Koss Pro4AA (1973). My preferred can for listening to rap. Very powerful bass that reaches just as low as #1&2, less treble and mids. Isolates like no other, and doesn't leak sound, either. When fed lots of power, can play almost impossibly loud without distortion. Hot and uncomfortable, but just really sounds incredible when I'm listening to rap. Great for classic rock as well.
4: Superlux HD681. The only one out of this group that can be driven to really satisfying volume by my iPhone 4s. They're less comfortable than #1&2, but more than 3. Open enough to feel safe to use outside without missing something dangerous. Superb for the price, but I think that I find their response peculiarities less offensive than some people. Tyll's review of these was much more critical than I would be. These were the gateway drug that led to the DT990 purchase, and do have a similar sound with less comfort. In spite of the weaknesses, I'd consider them probably the best you can get for $35.
It might be a sacrilege around here, but I've modified all 4 to accept cables with 3.5mm ends so that they can be used as headsets on the phone. While not perfect for critical listening, I have to realistically accept that it's convenient to listen that way and so I adapted the headphones to that. I can still use a heavier cable at home when it's possible with stronger sources.
List of what's next on my list to try (and I'm open to suggestions, especially under the $100 mark):
AKG Q/K701
Beyerdynamic DT770
Pioneer SE-A1000
Noontec Zoro HD
Grado SR80e
Once I swap these out for some cloth pads the A900's will win all around.